1. Field of the Invention
This invention relates to maximum likelihood detection and more particularly relates to maximum likelihood detection for an asymmetric input signal.
2. Description of the Related Art
During a read process of a data storage device, a read head is typically passed over a data record recorded on a storage media in order to convert the recorded data into an analog signal. For example, in a magnetic tape drive, a magneto-resistive read head (“MRRH”) is passed over a data record that has been previously written as flux reversals on a magnetic tape. As the MRRH is passed over the tape, the MRRH converts the flux reversals into an electrical analog signal that represents the data originally stored on the magnetic tape.
An analog to digital converter (“ADC”) periodically samples the analog signal and converts the sampled analog signal to a digital input signal. The ADC may sample and convert a plurality of digital input signals to create a digital waveform. The amplitude of the digital waveform is processed to form binary values. The data storage device such as a magnetic media storage device processes the digital waveform to reconstruct the data that was originally written to the tape.
Unfortunately, because of noise in the read channel, a binary value corresponding to one or more digital input signals may be indeterminate. For example, one or more of the digital input signals may have a lower amplitude, which is corrupted by noise, increasing the difficulty of reconstructing the data and determining the binary value.
Storage devices such as magnetic tape drives frequently use a maximum likelihood detector to reconstruct data from the digital waveform. The maximum likelihood detector employs a plurality of states organized as a logical trellis. The trellis specifies allowable subsequent states for each state. For example, the trellis may specify that the detector may change from a first state to a second or a third state, but not to a fourth state.
The detector changes from the first state to the second state or the third state in response to a plurality of branch metrics. A branch metric for the current state is a function of branch metrics for previous states and a current digital input signal. Path metrics maintain a record of the detector's progression through previous states. The detector begins in an initial state, and proceeds to change from state to state in response to the branch metrics, identifying the binary value for one or more digital input signals using information about previous and subsequent digital input values from the path metrics. As a result, the detector is better able to reconstruct data from digital input values.
Unfortunately, read head wear, read head asymmetries, and the like may produce an asymmetric analog signal. For example, the analog signal typically comprises alternating positive voltage and negative voltage pulses. Positive pulses may have a greater pulse amplitude than negative pulses or negative pulses may have a greater pulse amplitude than positive pulses if, for example, a read head is worn. Yet the typical maximum likelihood detector is not designed for the asymmetric digital waveform, increasing the probability that the detector will interpret the asymmetric digital input signals incorrectly and resulting in an increased number of data errors.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method for maximum likelihood detection of asymmetric digital input signals. Beneficially, such an apparatus, system, and method would reduce data errors for read channels with asymmetries.